The invention relates to a method for determining the friction torque of a test bearing, wherein one race member of the test bearing is rotated at a constant rotational speed, while the other race member is connected via a measuring head to a spring acting as a force pickup which generates a signal as a measure of the friction torque of the test bearing.
In the constructions comprising roller bearings, the friction torque is an essential characteristic. Precise knowledge of the friction torque of the bearing utilized in aerospace applications where, for example, in the event of failure of a solar generator drive mechanism, the power available for the satellite is drastically reduced and, consequently, the success of the entire mission is jeopardized, is absolutely essential.
Besides the mean value, in particular, friction torque peaks are of significance. On the one hand, if the driving torque of the motor is exceeded, they can result in failure of the mechanism, on the other hand, such dynamic quantities influence the control behavior of the tracking electronics. Moreover, knowledge of such dynamic quantities and the effects on which they are based, is of interest in elucidating the failure mechanisms of the bearings and, consequently, in fundamental research on the lubrication systems used.
To guarantee the quality of such bearings it is, therefore, necessary to be able to make a calibrated measurement of the friction torque with high time resolution.
A large number of devices for measuring torques and forces is known. Usually, the friction torques are measured reactively via the deformation of an elastic element (spring), with the friction torque being proportional to the deformation of the spring in the static case.
The moment of inertia of the measuring head and the internal damping of the spring material do, however, result in an oscillatory system wherein the friction torque of the measurement bearing and the spring deflection are, as a rule, no longer proportional. For this reason, the strong damping case (aperiodic limiting case) where proportionality between friction torque and spring deformation is attained after a setting time, is usually realized. Such aperiodically damped systems are easy to calibrate using calibrated weights, however, their setting time is too long for many applications.
To reduce the setting time, it is known to measure not only the spring deformation, but also the terms proportional to the acceleration of the test pickup and to the speed of the test pickup (acceleration term and friction term). Using the complete oscillation equation of the test system, the friction torque is then determinable as a function of time. Such a method does indeed result in sufficiently short setting times, but calibration of damping term and acceleration term involves extremely high expenditure and is difficult to survey since no dynamic force standards exist. Such a method has already been described for dynamic disturbance elimination in balances (U. Milz in VDI-Berichte (Association of German Engineers Reports) No. 312, 1978, page 135 et seq.).
The object underlying the invention is to so improve a generic method that the friction torque of a bearing is determinable using extremely simple and precise calibration with high time resolution.